1. Field of the Invention
This invention relates to a cold-rolled ferritic stainless steel sheet having excellent deep-drawability, brittle resistance to secondary processing, compatibility with overcoating, and corrosion resistance suitable for use in outer panels and strengthening members of automobiles and the like. The invention also relates to a method for making the cold-rolled ferritic stainless steel sheet.
2. Description of the Related Art
Generally, outer panels and strengthening members of automobiles are made by press-forming high tensile strength steel sheets of a 440 Mpa class. Such steel sheets are generally subjected to surface treatment, such as plating, before working or to coating treatment after working to improve the corrosion resistance. In actual operation, however, when plated steel sheets are worked, they suffer from peeling of plated material. Such peeling causes rust to occur, which is a problem. Coating treatment after working cannot completely cover the minute details of complicated shapes. Rust occurs in the uncoated minute portions, which is a problem. Stainless steel sheets having high corrosion resistance are preferably used to prevent generation of rust resulting from insufficient plating or coating or the like. Austenitic stainless steel sheets, such as SUS 304, which contain a large amount of expensive nickel as a component, are themselves expensive. Hence, the cost is high compared with conventional coated steel sheets. In contrast, although ferritic stainless steel sheets are relatively inexpensive, they have low workability, e.g., low press-formability, and improvements as to this point are required.
In conventional technologies, improvement in workability, i.e., deep-drawability, and more specifically, an increase in r-value, of ferritic stainless steel sheets has been achieved by increasing the annealing temperature of cold-rolled sheets to promote the development of the {111} recrystallization structure effective for increasing the r-value, thereby increasing the ductility and the r-value. Japanese Unexamined Patent Publication No. 9-241738 discloses a technology whereby after carbon and nitrogen in the steel are decreased to 100 ppm or less, the remaining carbon and nitrogen are fixed as deposits by a carbide/nitride forming element such as Ti or Nb, and boron (B) is added to the steel to make ferritic stainless steel sheets having highly balanced ductility and r-value.
However, stainless steel sheets must have a higher deep-drawability to be press-formed into complicated shapes such as those required by outer panels or strengthening members of automobiles. The r-value of the conventional ferritic stainless steels has been 1.8 at most. However, the average r-value should be increased to 2.0 or more to be effective.
Workability, such as deep-drawability, can be improved by reducing solid-solution carbon and nitrogen and by adding boron, as described above. For example, stainless steel is formed into fuel tanks or the like. The resulting stainless steel products to which high strain is applied during a drawing process suffer from brittle fracture when an external force is applied thereto such as by flying stones or collision, for example. This is called brittleness to secondary processing. The brittle resistance to secondary processing indicates the brittle resistance to an external force applied to a deep-drawn product. This property is of a particular importance in cold climates such as northern North America, e.g., Alaska.
The deep-drawability, and more specifically the r-value, of ferritic stainless steel sheets has been improved by increasing the annealing temperature of the cold-rolled sheets to promote the development of the {111} recrystallization structure effective for increasing the r-value and to thereby increase the ductility and the r-value, as described above. However, high-temperature annealing increases the size of crystal grains of cold-rolled annealed sheets, thereby roughening the surface after working and decreasing the brittle resistance to secondary processing. Although Japanese Unexamined Patent Publication No. 9-241738, etc., disclose adding boron, as described above, no reference is made regarding the brittle resistance to secondary processing. The technology disclosed in Japanese Unexamined Patent Publication No. 9-241738 cannot achieve both high deep-drawability, i.e., the r-value of 2.0 or more, and high brittle resistance to secondary processing in cold climates, e.g., at an ambient temperature of −60° C.
No ferritic stainless steel sheets having both excellent deep-drawability and high brittle resistance to secondary processing has been developed. These two properties must be simultaneously achieved for the ferritic stainless steel sheets to be used as outer panels or strengthening members of automobiles or the like.
It is accordingly an object of the invention to achieve an r-value of 2.0 or more (deep-drawability) and a brittle resistance to secondary processing free of longitudinal cracking in a drop weight test at a low-temperature of −60° C. or less simulating the ambient environment of automobiles and the like.
When components made of ferritic stainless steel are used in coastal areas or districts where salt is used to melt snow and ice, the components may suffer from a decrease in brittle resistance to secondary processing and in corrosion resistance due to salt, even though the ferritic stainless steels generally have superior corrosion resistance. To overcome this problem, the components may be provided with a light coating or the like to further enhance the brittle resistance and the corrosion resistance and to widen the applicable range of ferritic stainless steels. Thus, it is another object of the invention to develop a coated steel which can be suitably used in such conditions.